1. Field of the Invention
According to a general aspect thereof, the present invention relates to a tyre for vehicles, and in particular to its tread pattern.
2. Description of the Related Art
As is known, a tyre in its most general form comprises a torus-shaped carcass with a central crown and two side walls terminating in a pair of beads for fixing the tyre onto a corresponding mounting rim, a tread band coaxially extending around the crown and a belt structure arranged between the carcass and the tread band.
On its external surface the tread band comprises a raised pattern which, together with the physico-chemical properties of the compound from which it is made, is especially suitable for providing the road-handling properties of the tyre, in particular the tractional power.
Generally the tread pattern is formed by several circumferentially and/or transversely directed grooves which result in voids and solid areas of rubber of different shapes and sizes which are usually called xe2x80x9cblocksxe2x80x9d.
As is known, in the sector of tyre production, a long-standing requirement has been that of ensuring an adequate performance of the tyre, despite the extremely high stresses to which it is subject during use, in particular when it is of the racing type.
Therefore the Applicants have considered the problem of obtaining a tread which is able to optimize the driving performance and grip.
U.S. Pat. No. 4,641,696 describes a tread of the directional type which is suitable for high-power vehicles and for extreme conditions of use. The tread pattern consists of a plurality of blocks which have a substantially rhomboidal shape and are arranged in at least six circumferential rows each other separated by straight circumferential grooves and by slanting transverse grooves.
The Applicants consider that the difficulty of fully satisfying the abovementioned requirement with the type of tread described is essentially related to the difficulty of limiting the mobility of the blocks present on the tread band as the blocks warm up during use.
Hitherto the attempts made in the art to ensure the desired high performance levels of the tyre, based on the design of tread bands provided with grooves having different inclinations and blocks of different shapes, have been unable to achieve fully satisfactory results.
During rolling of the tyre, in fact, the blocks present on the tread band are subjected to a whole series of thermo-mechanical stressesxe2x80x94mainly due to the heating of the compound as a result of friction and to the compressive and shearing forcesxe2x80x94which tend to cause flexing and deformation of the blocks. These stresses modify the geometry of the blocks and cause a decline in the performance of the tyre, in particular during the so-called xe2x80x9cextremexe2x80x9d driving conditions.
Another problem arises from the presence of straight circumferential grooves on the tread pattern since said latter grooves represent concentrated zones which are subject to deformation under high transverse loads that give rise to a xe2x80x9ccircumferential hingexe2x80x9d effect. Therefore said grooves modify the tyre footprint when drifting and therefore the grip.
The Applicants have considered in particular the problem of finding a tread pattern which is suitable both for dry and wet grounds.
A tread band, or a portion thereof, is characterized by the value of the xe2x80x9csolid/voidxe2x80x9d ratio which depends on the quantity of rubber which is removed from the tread band owing to the presence of the grooves.
High values of the xe2x80x9csolid/voidxe2x80x9d ratio result in good tractional properties on dry ground, but low road-holding values on wet ground.
Numerous examples of tread patterns which have been especially designed to achieve optimum performance of the tyre on dry and wet ground are already known.
For example, U.S. Pat. No. 4,700,762 describes a tyre provided with a tread pattern formed by a large-width circumferential groove which is located along the equatorial plane and by a plurality of grooves which are extended substantially in the transverse direction on either side of the central groove up to the shoulders of the tyre.
Each transverse groove forms an angle of 45xc2x0 at the point where it meets with the central groove and then gradually curves until it assumes an orientation substantially perpendicular to the equatorial plane in the end zone of the tread on the shoulder of the tyre.
Therefore, the central groove and the plurality of transverse grooves produce on the sides of the equatorial plane two series of blocks each one extending from the centre to the shoulders of the tyre.
In a further embodiment, the tread pattern has a plurality of circumferential grooves which are parallel to the central groove and which intersect the transverse grooves.
Said further embodiment results in a plurality of circumferential adjacent rows of rhomboidal blocks on each side of the equatorial plane.
The tread pattern is designed so that three to five transverse grooves are present underneath the tyre footprint, each of said grooves being sized in order not to close when the tread comes into contact with the ground; therefore, as a result of said geometric features, the water which is accumulated in the centre of the tyre footprint is conveyed through the transverse grooves towards the shoulders of the tyre and here discharged ensuring driving stability and tractional properties even on wet surfaces.
A tyre marketed by the Applicants under the name xe2x80x9cP ZERO Cxe2x80x9d and comprising a tread pattern formed by two distinct parts which are each other circumferentially located alongside, i.e. an external part and an internal part with respect to the mounting position of the tyre on the vehicle, is known too.
The external part of the tread comprises, from the central zone towards the corresponding end, a continuous longitudinal rib which is defined between two circumferential grooves and comprises on both sides substantially axial incisions partially extending in the direction of the width of the rib. In an axially external position, the abovementioned rib has, arranged alongside it, another shoulder-type rib which is substantially smooth, except for an ordered series of small blind incisions extending circumferentially in succession on the internal side.
Said first part of the pattern has a very high solid/void ratio so as to ensure good tractional properties on dry ground.
The second part of the tread, i.e. the internal part, comprises, from the centre towards the corresponding end, a longitudinal rib defined between two central grooves and cut by transverse grooves which are directed substantially in the form of a herring-bone pattern and, in a more axially external position, it comprises a row of substantially rhomboidal shoulder blocks.
Said second part of the pattern has a solid/void ratio which is lower than that of the first part so as to provide good tractional properties also on wet ground.
It should also be pointed out that in the market there is a great demand for tyres with optimum performance features both on dry and on wet ground in order to satisfy a certain category of clients who own powerful cars in particular for use on racing tracks or in any case for racing purposes.
The Applicants, in the light of the state of the art described above, have set themselves the task of realizing a tread provided with a pattern which is suitable both for dry and wet grounds and having features in order to ensure driving stability not only to cars intended for ordinary use but also to powerful cars used for racing purposes and the like.
The Applicants have thought to specialize the tread pattern by using a system of grooves intended for a specific purpose (in place of the known multi-purpose grooves), some of which are specifically suitable for providing an optimum performance on wet ground, while others, different from the first ones, are suitable for ensuring an optimum performance on dry ground. The Applicants have realized that it was possible to solve the problem by using any combination of grooves on condition that they were distributed in such a way that they did not intersect each other, avoiding also the formation of circumferential grooves, all of which with the aim of eliminating isolated blocks and zones with concentrated compliance (circumferential hinges), fact which causes a certain mobility underneath the tyre footprint and consequent unstable driving conditions.
Therefore it has been thought that a convenient solution could consist in an arrangement of grooves such that it were possible to pass from one shoulder to the opposite shoulder of the tyre without passing through one or more grooves.
It has also been found that the performance of the tyre on wet ground could be improved by using one or also several series of xe2x80x9cblindxe2x80x9d grooves, the longitudinal extension of which were greater than the length of the tyre footprint inflated to the nominal operating pressure and subjected to the nominal load, under static conditions.
Hereinbelow the wording xe2x80x9cblindxe2x80x9d grooves indicates that said grooves are cavities which are surrounded by continuous walls and hence without entrance or exit paths.
It has also been found to be convenient to distribute the blind grooves along a central portion of the tyre footprint in which there is the greater risk of water stagnation without thereby excluding further blind grooves with an extension greater than the length of the tyre footprint in other portions of the tread.
In practice it has been found that the water collected by the blind grooves underneath the tyre footprint could flow towards the ends located outside the tyre footprint, preventing problems of aquaplaning.
According to a first aspect thereof, therefore the invention relates to a tyre for vehicle wheels which is provided with a tread pattern having solid areas extending in a continuous manner between two shoulders which are axially opposite with respect to the equatorial plane of the tyre.
The pattern comprises two lateral rows of grooves and at least a third central row formed by a plurality of grooves arranged between said lateral rows, the grooves of each row being circumferentially spaced from one another, and the grooves of the lateral rows axially extending from the corresponding shoulder as far as a predetermined distance from the equatorial plane.
The main features of the tread pattern are as follows:
all the grooves of the three rows are separate from one another so as to produce a pattern with no communicating paths in the thickness of the tread which connect together said grooves;
the ends of the grooves of the third row are far from the shoulders of the tyre;
the maximum distance between two points of each groove of the third row, measured according to the circumferential direction, is greater than the length of the tyre footprint measured at the nominal inflation pressure and subjected to the nominal load under static conditions.
The greater dimension of each groove of the central row with respect to the length of the tyre footprint results in the drainage of the water underneath the tyre footprint.
Conveniently, the maximum distance between said two points of each groove of the third row is comprised between 1.01 and 2.5 times the length of the tyre footprint.
In a preferred embodiment the tyre comprises grooves of the third row which have a width, measured over the surface of the tread, ranging between 6 and 15 mm.
According to a first variation of embodiment, the tyre comprises an additional fourth row of grooves which are spaced from one another circumferentially and separate from those of the other rows, the grooves of said fourth row starting from a shoulder, between two adjacent grooves of a lateral row, and terminating between two adjacent grooves of the central row.
Said embodiment is particularly suitable for an xe2x80x9casymmetrical directionalxe2x80x9d tyre, where xe2x80x9casymmetricalxe2x80x9d is intended as a tread pattern having a portion which is different from the axially opposite portion, with respect to the equatorial plane of the tyre, and xe2x80x9cdirectionalxe2x80x9d is intended as a tread pattern which identifies a particularly preferred direction of rotation of the tyre.
Preferably in said tyre the grooves of a first lateral row together with the grooves of the third row and the grooves of the second lateral row together with the grooves of the fourth row are respectively aligned along a first and a second trajectory each other circumferentially alternated, said trajectories having a substantially undulating shape with the peaks of the undulations being aligned along circumferential planes parallel to the equatorial plane, said first trajectory being interrupted between the grooves of the first and the third row and said second trajectory being interrupted between the grooves of the fourth and the second lateral row.
In a further variation of embodiment, the tyre is characterized in that the grooves of said fourth row are symmetrical, relative to the equatorial plane, with those of said third row.
Conveniently said tyre is of the directional type and is characterized in that the axially most internal peaks of the substantially undulating trajectories comprising respectively said third and fourth rows of grooves are aligned on the equatorial plane.
According to a second aspect thereof, the invention relates to a tyre characterized in that it comprises means for acoustically signalling the value of the inflation pressure when it is lower than a predefined limit, said means consisting in the fact that the dimension, in the direction of travel, of one or more grooves of said central row is greater than the length of the tyre footprint, measured at the nominal inflation pressure and subjected to the nominal load, under static conditions. In this way, under pressure values lower than said predefined limit, the tyre footprint assumes dimensions which are at least equal to those of said grooves. This feature, during the rolling of the tyre, firstly causes the retention of the air in the grooves, during the contact with the ground, and then the immediate expulsion of the air outside the tyre footprint, with consequent generation of a fairly loud noise.
According to a further aspect thereof, the invention relates to a method for checking the correct value of the air pressure inside of a tyre for vehicle wheels provided with a tread pattern axially extending in a continuous manner between two shoulders which are axially opposite with respect to the equatorial plane of the tyre.
The method comprises the steps of:
a) forming in the tread pattern at least one row of grooves which are circumferentially spaced from one other;
b) assigning to at least some of the successive grooves of said row a dimension in the direction of travel which is greater than the dimension of the tyre footprint in the same direction, measured on the tyre at the nominal inflation pressure and subjected to the nominal load, under static conditions;
c) checking, under static load conditions with the inflation pressure less than the predefined value, whether the dimensions (length, width and depth) of the grooves are such as to allow the enclosure of the air trapped underneath the tyre footprint and the expulsion thereof outside the tyre footprint with the generation of noise;
d) modifying the dimensions and the number of said grooves until an acoustic signal indicating a lower air pressure inside the tyre is produced.
According to a fourth aspect thereof, the invention relates to an acoustic signalling device for vehicle wheels provided with a tread pattern comprising several groups of grooves including at least one axially internal row of grooves circumferentially spaced from each other, characterized in that it comprises one or more grooves of said internal row which, in the direction of rolling of the tyre, have a dimension which is greater than the dimension of the tyre footprint measured on the tyre at the nominal inflation pressure subjected to the nominal load under static conditions and a dimension smaller than or equal to the dimension of the tyre footprint measured on the tyre at an inflation pressure which is lower than a predefined limit subjected to the nominal load, under static conditions.
According to a fifth aspect thereof, the invention relates to a tread band for vehicle tyres, provided with a tread pattern defined between two shoulders which are each other axially opposite with respect to the equatorial plane of the tyre and comprising: at least a first and a second circumferential series of cavities longitudinally extending in the direction of forward travel of the tyre, where said first and second circumferential series of cavities define an essentially continuous portion of tread band axially extending between the two shoulders and are such as to produce a directional pattern characterized in that at least the ends of the cavities of said first circumferential series are far from said shoulders.
According to a different aspect thereof, the invention relates to a method for indicating the reduction in the inflation pressure of a tyre provided with a tread pattern, characterized in that it has at least one element of said tread pattern which is able to vary the noise level of the tread upon variation of the inflation pressure of said tyre.
According to the invention, the Applicants have found in particular that the abovementioned essentially continuous portions of tread band, which extend alternately from opposite shoulder zones towards the equatorial plane of the tyre form a kind of xe2x80x9cnetworkxe2x80x9d or xe2x80x9cmatrixxe2x80x9d of portions of elastomeric material which are each other interlocked and substantially devoid of longitudinal hinge elements.
Moreover said essentially continuous portions of tread band form a corresponding number of sets of xe2x80x9cstrutsxe2x80x9d which are suitable for transmitting, along their axis and towards the rear side of the essentially continuous and axially opposite portions, the stresses imparted to them during the rolling of the tyre.
The structural rigidity due to the interlocking of the essentially continuous and axially opposite portions of the tread band allows said portions to absorb all those thermo-mechanical stresses imparted to them during the rolling of the tyre without flexing or being excessively deformed.
Owing to said reduction in the mobility of the different tread band portions, a drastic reduction in the thermo-mechanical degradation phenomena of the elastomeric matrix of the tread band has been noted, even in the presence of very high stresses.